Semiconductor storage device and method of manufacturing the same

ABSTRACT

A method of manufacturing a semiconductor storage device having a capacitive element having a dielectric layer having a perovskite-type crystal structure represented by general formula ABO 3  and a lower electrode and an upper electrode disposed so as to sandwich the dielectric layer therebetween; in the method are carried out forming, on a lower electrode conductive layer, using a MOCVD method, an initial nucleus containing at least one metallic element the same as a metallic element in the dielectric layer, forming, on the initial nucleus, using a MOCVD method, a buffer layer containing at least one metallic element the same as the metallic element contained in both the initial nucleus and the dielectric layer, in a higher content than the content of this metallic element contained in the initial nucleus, and forming, on the buffer layer, using a MOCVD method, the dielectric layer having a perovskite-type crystal structure.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor storage device havingcapacitive elements and a method of manufacturing the semiconductorstorage device, and in particular relates to a method of forming adielectric film using a metal organic chemical vapor deposition method.

2. Description of a Related Art

In recent years, research and development has been carried out withvigor into ferroelectric memories that utilize the polarizationcharacteristics of a ferroelectric, DRAMs that use ahigh-dielectric-constant material, and so on.

As methods of forming a film of a ferroelectric or ahigh-dielectric-constant material, conventionally a sol-gel method, asputtering method, a CVD method and so on have been used. Of thesemethods, the CVD method is superior in terms of the uniformity of filmformation over a large-diameter wafer and the ability to cover oversteps on the surface, and hence is promising as an ULSI mass productiontechnique.

As methods of forming a film of a ferroelectric or ahigh-dielectric-constant material using such a CVD method, in particulara metal organic chemical vapor deposition (MOCVD) method, the followingmethods have been disclosed.

Japanese Unexamined Patent Application Publication No. 2000-58525discloses a vapor phase deposition method for a metal oxide dielectricfilm (in particular a PZT film) having a perovskite-type crystalstructure represented by general formula ABO₃, the deposition methodhaving a step of forming an initial nucleus or initial layer having aperovskite-type crystal structure on a conductive material under firstfilm formation conditions, and a step of further forming a film having aperovskite-type crystal structure on the initial nucleus or initiallayer under second film formation conditions. Furthermore, it is statedthat the raw material supply amounts for the constituent elements arechanged between the first film formation conditions and the second filmformation conditions. Moreover, it is disclosed that according to thismethod, the film formation can be carried out at a low temperature (450°C. or less) so as not to cause degradation of plugs, wiring andtransistors in a lower layer, and moreover a metal oxide dielectric filmhaving excellent orientation and crystallinity can be formed.

Japanese Unexamined Patent Application Publication No. 2002-334875discloses a vapor phase deposition method for a metal oxide dielectricfilm (in particular a PZT film) having a perovskite-type crystalstructure represented by general formula ABO₃, the deposition methodhaving a step of forming an initial nucleus or initial amorphous layerhaving a perovskite-type crystal structure on a conductive materialunder first film formation conditions, and a step of further forming afilm having a perovskite-type crystal structure on the initial nucleusor initial amorphous layer under second film formation conditions, andcharacterized in that here the first film formation conditions satisfyat least one of the substrate temperature being lower than for thesecond film formation conditions and the raw material gas pressure beinghigher than for the second film formation conditions. Furthermore, it isstated that according to this method, a dielectric film is formed forwhich the leakage current is low and moreover the transparency isexcellent and hence mask alignment can be carried out well, andfurthermore if this dielectric film is used in a capacitive element thena semiconductor device having little variation in bit line voltagedifference can be manufactured.

However, with the method of forming a dielectric film using the MOCVDmethod having two steps as in the conventional art described above,there have been calls for further improvements with regard to thecapacitance characteristics of the element formed.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the present invention toprovide a semiconductor storage device having capacitive elements forwhich capacitance characteristics, in particular low voltagecharacteristics, are improved, and a method of manufacturing thesemiconductor storage device.

To these ends, according to one aspect of the present invention, thereis provided a method of manufacturing a semiconductor storage devicehaving a capacitive element having a dielectric layer having aperovskite-type crystal structure represented by general formula ABO₃and a lower electrode and an upper electrode disposed so as to sandwichthe dielectric layer therebetween, the method comprising forming, on aconductive layer constituting the lower electrode, using a MOCVD method,an initial nucleus containing at least one metallic element the same asa metallic element in the dielectric layer; forming, on the initialnucleus, using a MOCVD method, a buffer layer containing at least onemetallic element the same as the metallic element contained in both theinitial nucleus and the dielectric layer, in a higher content than thecontent of this metallic element contained in the initial nucleus; andforming, on the buffer layer, using a MOCVD method, the dielectric layerhaving a perovskite-type crystal structure.

According to another aspect of the present invention, there is provideda semiconductor storage device having a capacitive element having adielectric layer having a perovskite-type crystal structure representedby general formula ABO₃ and a lower electrode and an upper electrodedisposed so as to sandwich the dielectric layer therebetween, thesemiconductor storage device having, on conductive layer constitutingthe lower electrode, an initial nucleus containing at least one metallicelement the same as a metallic element in the dielectric layer; andbetween the initial nucleus and the dielectric layer, a buffer layercontaining at least one metallic element the same as the metallicelement contained in both the initial nucleus and the dielectric layer,in a higher content than the content of this metallic element containedin the initial nucleus.

The present invention thereby provides a semiconductor storage devicehaving capacitive elements for which the capacitance characteristics, inparticular the low voltage characteristics, are improved.

The above and other objects, features and advantages of the presentinvention will become more fully understood from the detaileddescription given hereinbelow and the accompanying drawings which aregiven by way of illustration only, and thus are not to be considered aslimiting the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the capacitor structure of memory according to a specificembodiment of the invention.

FIGS. 2A and 2B show the hysteresis characteristics for the capacitiveelement of an example and a comparative example, respectively.

FIGS. 3A and 3B show the hysteresis characteristics for the capacitiveelement of an example and a comparative example, respectively.

FIGS. 4A and 4B show the hysteresis characteristics for the capacitiveelement of an example and a comparative example, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be describedhereinafter with reference to the drawings.

The present invention relates to the formation of a dielectric layer ofa capacitive element using a metal organic chemical vapor deposition(MOCVD) method, and is principally characterized by forming, on aconductive layer, an initial nucleus containing at least one metallicelement the same as a metallic element in the dielectric layer, and thenproviding, on the initial nucleus, a buffer layer containing at leastone metallic element the same as the metallic element contained in boththe initial nucleus and the dielectric layer, in a higher content thanthe content of this metallic element contained in the initial nucleus,and forming the dielectric layer on the buffer layer. The presentinvention can provide a semiconductor storage device having capacitiveelements for which the capacitance characteristics, in particular thelow voltage characteristics, are improved.

The present invention is effective in the case that a dielectriccontaining a metallic element that evaporates relatively easily underthe temperature and pressure conditions in the film formation process,i.e. a metallic element having a high vapor pressure (hereinafterreferred to as a ‘high-vapor-pressure metallic element’), in particulara dielectric containing lead (Pb) (hereinafter referred to as a‘Pb-based dielectric), is used as the material of the dielectric layer.

Conventionally, when forming a dielectric layer of a capacitive element,to improve the orientation and crystallinity and the inversional fatigueresistance, a first step of forming an initial nucleus on a conductivelayer, and a second step of forming a dielectric layer having aperovskite-type crystal structure on the initial nucleus have beencarried out. The inventors of this invention carried out assiduousstudies from the viewpoint of improving the capacitance characteristics,and as a result discovered that with this method there are the followingproblems. That is, the film formation conditions (raw material supplycontent, temperature, pressure, etc.) differ between the first step andthe second step, and hence when switching the film formation conditionsbetween the first step and the second step, a waiting period is requireduntil the film formation conditions stabilize at the prescribedconditions for the second step. Consequently, when forming a dielectriclayer containing a high-vapor-pressure metallic element as describedabove as the dielectric layer, in the case of forming an initial nucleuscontaining a metallic element the same as the high-vapor-pressuremetallic element, depending on the length of the waiting period and thevarious conditions, the high-vapor-pressure metallic element is prone toevaporating, causing a defect at the surface of the initial nucleus. Ifstoichiometric defects at the surface of the initial nucleus becomemarked, then there will be an adverse effect on the crystal orientationof the dielectric layer formed thereupon, and this worsening of thecrystal orientation and the stoichiometric defects will cause aworsening of the capacitance characteristics.

The present invention thus aims at providing a semiconductor storagedevice having capacitive elements with improved capacitancecharacteristics by providing, on the initial nucleus, a buffer layerthat contains a metallic element the same as a high-vapor-pressuremetallic element contained in the initial nucleus, wherein the contentof this metallic element in the buffer layer is greater than the contentof the same contained in the initial nucleus, thus filling defects, andthen providing a dielectric film on the buffer layer.

Following is a description of each of the constituent elements of thecapacitive element in the present invention.

[Electrodes]

In the present invention, as a lower electrode and upper electrode thatare disposed so as to sandwich the dielectric layer therebetween,electrodes having platinum (Pt), iridium (Ir), iridium oxide (IrO₂),ruthenium (Ru), ruthenium oxide (RuO, RuO₂), gold (Au), titanium nitride(TiN) or the like as a principal component thereof can be used. Theseelectrodes can be formed using CVD, sputtering, vacuum deposition or thelike. In the present invention, the lower electrode and the upperelectrode preferably each have, at least on a surface on the dielectriclayer side, a film comprising at least one material selected fromplatinum (Pt), iridium (Ir), ruthenium (Ru), and oxides thereof.

[Initial Nucleus]

In the present invention, the initial nucleus is provided on theconductive layer constituting the lower electrode, and contains at leastone metallic element the same as a metallic element in the subsequentlyformed dielectric layer. By forming the dielectric layer after providingsuch an initial nucleus, compared with the case that the dielectriclayer is provided directly on the conductive layer, a dielectric layerhaving better orientation, crystallinity, and inversional fatigueresistance can be formed. From the viewpoint of obtaining yet bettercharacteristics, the initial nucleus is preferably a crystalline nucleuscomprising an element A, element B and oxygen, and more preferably has aperovskite-type crystal structure represented by general formula ABO₃.

Regarding the constituent elements of the initial nucleus, the initialnucleus may contain all of the same metallic elements as the metallicelements in the subsequently formed dielectric layer, or may contain oneor some of these metallic elements. From the viewpoint of thecontrollability of the film formation conditions, the crystallinity andso on, the metallic elements in the initial nucleus are preferablyselected from the metallic elements in the dielectric layer. Forexample, in the case that the subsequently formed dielectric layer is tobe a PZT layer (containing Pb as element A, and Zr and Ti as elements B,the initial nucleus is preferably a PZT layer or a lead titanate (PTO)layer, and from the viewpoint of the controllability of the filmformation conditions, the crystallinity and so on, is particularlypreferably a PTO layer.

From the viewpoint of the capacitance characteristics, the ratio B/A ofthe element B to the element A in the initial nucleus (in the case oflead titanate, Ti/Pb) is preferably at least 0.5, more preferably atleast 0.8, and on the other hand is preferably not more than 1.5, morepreferably not more than 1.2, and is particularly preferably in a rangeof 0.9 to 1.1.

In the present invention, the initial nucleus may be made to be acontinuous film covering the whole of the upper surface of the lowerelectrode, but from the viewpoint of suppressing a bias in the electricfield and a drop in the dielectric constant, and the controllability ofthe crystal grain size, the initial nucleus preferably comprises aplurality of islands formed at high density. Moreover, from theviewpoint of the capacitance characteristics and so on, the thickness ofthe initial nucleus is preferably at least 1 nm, more preferably atleast 2 nm, and on the other hand is preferably not more than 10 nm.

The processing time in the formation of the initial nucleus (first step)can be set as appropriate from, for example, a range of 5 to 60 seconds.If the processing time is too short or too long, then it will becomedifficult to obtain a dielectric film having the desiredcharacteristics.

[Buffer Layer]

In the present invention, the buffer layer is provided on the initialnucleus that has been provided on the conductive layer constituting thelower electrode, and the dielectric film is provided on this bufferlayer. The buffer layer must contain at least one metallic element thesame as a metallic element contained in both the initial nucleus and thedielectric layer, in a higher content than the content of this metallicelement contained in the initial nucleus. It is preferable for thecontent of at least a high-vapor-pressure metallic element contained inthe buffer layer to be greater than the content of the same contained inthe initial nucleus.

In a film formation process using the MOCVD method, in the case that theinitial nucleus contains a high-vapor-pressure metallic element, if thewaiting period between the step of forming the initial nucleus and thestep of forming the dielectric layer is long, then thishigh-vapor-pressure metallic element will be prone to evaporating fromthe surface of the initial nucleus, causing a defect. Stoichiometricdefects due to this defect may cause a worsening of the capacitancecharacteristics. With the present invention, a buffer layer thatcontains this high-vapor-pressure metallic element which is likely tocause a defect in a higher content than in the initial nucleus isprovided on the initial nucleus, and then the dielectric film is formed.As a result, compared with the case that such a buffer layer is notprovided, the capacitance characteristics, in particular the low voltagecharacteristics, can be improved.

From the viewpoint of the capacitance characteristics, the waitingperiod between the step of forming the initial nucleus and the step offorming the buffer layer, and the ease of operation, it is preferablefor a metallic element in the buffer layer to be selected from themetallic elements in the initial nucleus. Moreover, from the viewpointof the device characteristics, the buffer layer preferably contains atleast one of the element A and one of the element B. For example, in thecase that the initial nucleus contains lead (Pb), it is preferable forthe buffer layer to contain Pb, and for the Pb content in the bufferlayer to be greater than that in the initial nucleus. In the case thatthe dielectric layer comprises PZT and the initial nucleus compriseslead titanate, the buffer layer can be constituted from lead titanate orlead oxide. From the viewpoint of the capacitance characteristics, leadtitanate is more preferable.

The content of a metallic element for which one wishes to prevent lossfrom the surface of the initial nucleus, in particular the content of ahigh-vapor-pressure metallic element, in the buffer layer can be set asappropriate from within a range higher than the content of the same inthe initial nucleus. If the content of this metallic element in thebuffer layer is too low, then the desired capacitance characteristicimprovement effect will no longer be obtained. Conversely, if thiscontent is too high, then the capacitance characteristic improvementeffect will tend to drop. In the case of forming, for example, leadtitanate as the initial nucleus, by forming lead oxide as the bufferlayer, a capacitance characteristic improvement effect can be obtained.Moreover, in the case of forming lead titanate as the initial nucleusand the buffer layer, if the Ti/Pb ratio in the initial nucleus isaround 1, for example 0.8 to 1.2, more preferably 0.9 to 1.1, then bysetting the Ti/Pb ratio in the buffer layer to be within a range of, forexample, 0.2 to 1, more preferably 0.4 to 0.8, a higher capacitancecharacteristic improvement effect can be obtained.

The thickness of the buffer layer is preferably at least 0.2 nm, morepreferably at least 0.4 nm, yet more preferably at least 1 nm, and onthe other hand is preferably not more than 10 nm, more preferably notmore than 8 nm, yet more preferably not more than 5 nm. If the bufferlayer is too thin, then it will no longer be possible to obtain asufficient capacitance characteristic improvement effect. Conversely, ifthe buffer layer is too thick, then the influence of the buffer layer onthe crystal orientation of the dielectric layer formed thereon willbecome large, and hence the capacitance characteristics may worsen. Thebuffer layer thus preferably has a thickness such as not to exert aninfluence on the crystal orientation of the dielectric layer.

For the buffer layer, two or more layers may be built up on top of oneanother; in this case, the layers can be built up so that the content ofthe high-vapor-pressure metallic element such as Pb becomesprogressively greater from the side of the lower electrode toward thedielectric layer. Moreover, the buffer layer may be a layer having acomposition distribution such that the content of thehigh-vapor-pressure metallic element in the dielectric layer increasescontinuously from the side of the lower electrode toward the dielectriclayer.

[Dielectric Layer]

In the present invention, the dielectric layer has a perovskite-typecrystal structure represented by general formula ABO₃ and is provided onthe buffer layer; the upper electrode is provided on the dielectriclayer. The thickness of the dielectric layer can be set as appropriatefrom within a range of, for example, 50 to 500 nm.

In the present invention, the dielectric layer preferably comprises adielectric containing Pb as an element A (a Pb-based dielectric), with aferroelectric having a perovskite-type crystal structure containing lead(Pb) as an element A occupying the A lattice site and containingzirconium (Zr) and titanium (Ti) as elements B occupying the B latticesite (hereinafter referred to as ‘PZT’) being particularly preferable.As this PZT, one represented by the general formula (Pb_(1-x)M_(x))(Zr_(y)Ti_(1-y))O₃, wherein x and y are in the ranges 0≦x<1 and 0<y<1respectively, can be used. An example of M in the formula is at leastone selected from Nb, La, Li, Na, Mg, Ca, Sr, Ba and Bi. From theviewpoint of obtaining the desired device characteristics, it ispreferable for x in the formula to be within the range 0≦x≦0.2 and it isparticularly preferable for x to be 0, i.e. for the dielectric to be onerepresented by general formula Pb(Zr_(y)Ti_(1-y))O₃. From the viewpointof obtaining the desired device characteristics, in particularsuppressing the leakage current, y in the formula is preferably at least0.3, more preferably at least 0.35, and on the other hand from theviewpoint of obtaining a sufficient remanent polarization, y ispreferably not more than 0.8, more preferably not more than 0.7.

In accordance with the desired characteristics, the dielectric layer maybe made to have a varying composition in the thickness direction, or maybe made to comprise dielectric layers having different compositionsbuilt up on top of one another.

[Method of Forming Films by MOCVD]

Following is further description of the method of forming the initialnucleus, the buffer layer and the dielectric layer using MOCVD. Theformation of these layers can be carried out using a publicly knownvapor phase deposition apparatus for MOCVD.

The organometallic raw materials used in the MOCVD can be vaporized bybeing heated, and supplied into a vacuum vessel (deposition vessel) inwhich a substrate has been placed, if necessary together with a carriergas. Organometallic raw materials are often solids or liquids at normaltemperature and normal pressure. A solid raw material can be suppliedusing a solid sublimation method, or a liquid transport method in whichthe solid raw material is dissolved in a suitable solvent, the liquid istransported, and vaporization is carried out using a vaporizerimmediately before introduction into the vacuum vessel. A liquid rawmaterial can be supplied using the liquid transport method, either as isor if necessary after having been diluted using a solvent.

The vaporized raw materials (raw material gases) are supplied onto thesubstrate which has been heated to a prescribed temperature in thevacuum vessel which is held under reduced pressure, thus carrying outfilm formation. At this time, from the viewpoint of controlling the rawmaterial gas composition content, the temperature of the inner walls ofthe vacuum vessel and the raw material supply system is preferablycontrolled to be at least a temperature at which the raw materials havea sufficient desorption velocity (vapor pressure) such as not toaggregate on the inner walls but less than a temperature at which theraw materials decompose. For example, this temperature can be set toapproximately 180 to 220° C.

As the organometallic raw materials, for example in the case of PZT,lead bis-dipivaloylmethanate (Pb(DPM)₂) can be used for the Pb, titaniumisopropoxide (Ti(OiPr)₄) or titanium diisopropoxybis-dipivaloylmethanate (Ti(OiPr)₂(DPM)₂) can be used for the Ti, andzirconium butoxide (Zr(OtBu)₄) or zirconium isopropoxytris-dipivaloylmethanate (Zr(OiPr)(DPM)₃) can be used for the Zr.

To prevent formation of an alloy or oxygen defects on the conductivelayer constituting the lower electrode, it is preferable to supply in anoxidizing gas together with the organometallic raw material gases.Examples of this oxidizing gas are nitrogen dioxide (NO₂), ozone,oxygen, oxygen ions, and oxygen radicals; of these, nitrogen dioxide,which has high oxidizing ability, is preferable.

Further description will now be given, taking as an example the case offorming an initial nucleus and a buffer layer comprising lead titanateand a ferroelectric layer comprising PZT using these raw material gases.

First, a substrate on which a lower electrode conductive film has beenformed is installed in the vacuum vessel. The pressure inside the vacuumvessel is then held at a prescribed reduced pressure, and the substratetemperature is held at, for example, not more than 450° C. The filmformation conditions in the manufacturing method of the presentinvention do not necessarily have to be constant throughout the steps offorming the initial nucleus, the buffer layer and the ferroelectric film(the first, second and third steps); for example, as will be describedlater, it is possible to carry out the formation of the initial nucleusat a relatively low temperature, and carry out the formation of thedielectric layer at a temperature higher than the temperature for theformation of the initial nucleus, or carry out the formation of theinitial nucleus at a relatively high pressure, and carry out theformation of the dielectric layer at a pressure lower than the pressurefor the formation of the initial nucleus.

Next, the Pb raw material gas, the Ti raw material gas and the oxidizinggas are supplied at prescribed flow rates into the vacuum vessel for aprescribed time period, thus forming the initial nucleus on thesubstrate (first step). After that, the supply of the Pb raw materialgas, the Ti raw material gas and the oxidizing gas is suspended.

When forming the initial nucleus, it is preferable to carry out apretreatment step before this. For example, the Pb raw material gas issupplied at a prescribed flow rate into the vacuum vessel for aprescribed time period, and then while supplying in the Pb raw materialgas the oxidizing gas is supplied in at a prescribed flow rate for aprescribed time period (pretreatment step), and then in this statesupply of the Ti raw material gas is further commenced and this state isheld for a prescribed time period, thus forming the initial nucleus onthe substrate (first step). In the pretreatment step, the Pb rawmaterial gas is supplied onto the conductive layer either alone ortogether with the oxidizing gas. By carrying out this pretreatment step,a dielectric layer having a small grain size and hence having only smallsurface roughness can be formed in the subsequent step of forming thedielectric layer, and as a result a dielectric layer having a lowleakage current and excellent transparency can be formed. Thepretreatment step must be carried out for a time period and undertreatment conditions such that the Pb raw material gas decomposes on thesurface of the conductive layer and is able to react sufficiently withthe surface metal, but a PbO film is not formed on the conductive layer.For example, from the viewpoint of sufficiently obtaining the desiredeffects, the treatment temperature (the temperature of the conductivelayer) is preferably at least 350° C., more preferably at least 390° C.,but on the other hand from the viewpoint of suppressing PbO filmformation and suppressing thermal degradation of aluminum wiring and thelike, this treatment temperature is preferably not more than 700° C.,more preferably not more than 600° C., yet more preferably not more than450° C. The treatment time can generally be set as appropriate within arange of up to 60 seconds, for example 3 to 20 seconds. Note that PbOfilm formation can be investigated through X-ray diffraction.

Next, under flow rate conditions in which the flow rate ratio of the Pbraw material gas to the Ti raw material gas is greater than in the firststep, for example supplying the Ti raw material gas at a lower flow ratethan in the first step with the flow rates of the Pb raw material gasand the oxidizing gas being the same or substantially the same as in thefirst step, the buffer layer is formed (second step). At this time, ifnecessary, one or both of the temperature and the pressure may bechanged relative to the first step. After a prescribed time period haspassed, the supply of the Pb raw material gas, the Ti raw material gasand the oxidizing gas is suspended.

Next, changing the raw material supply conditions, the Pb raw materialgas, the Zr raw material gas, the Ti raw material gas and the oxidizinggas are each supplied at a prescribed flow rate, and this state is heldfor a prescribed time period, thus forming a ferroelectric layer of aprescribed thickness (third step). At this time, if necessary, one orboth of the temperature and the pressure may be changed relative to theprevious steps. Note that when carrying out the third step, from theviewpoint of preventing Pb loss, it is preferable for the waiting periodbetween the second step and the third step to be as short as possible.It is thus preferable for the conditions (temperature and pressure) inthe second step to be as close as possible to those in the third step.As a result, the time period required for stabilization after changingthe film formation conditions in preparation for the third step can beshortened, i.e. the waiting period can be shortened.

After the formation of the ferroelectric layer has been completed, aconductive layer for upper electrode formation is formed thereon bysputtering, CVD or the like.

[Film Formation Temperature and Pressure]

Throughout the first to third steps of the MOCVD in the manufacturingmethod of the present invention, from the viewpoint of crystallinity,the treatment temperature (the temperature of the conductive layer) ispreferably at least 350° C., more preferably at least 370° C. On theother hand, from the viewpoint of the heat resistance of the materialsand suppressing the leakage current, the treatment temperature ispreferably not more than 700° C., and in particular, considering theheat resistance of plugs and wiring comprising a low-melting-pointmaterial such as aluminum and preventing thermal degradation oftransistors, the treatment temperature is more preferably not more than450° C. Moreover, throughout the first to third steps, from theviewpoint of the film formation rate, the total pressure of the rawmaterial gases is preferably at least 1×10⁻⁴ Torr (1.33×10⁻² Pa). On theother hand, in the first step, from the viewpoint of the crystallinity,the total pressure can be set as appropriate from within a range of upto 100 Torr (13.3 kPa), for example can be made to be not more than 20Torr (2.66 kPa). In the third step, from the viewpoint of thecrystallinity, the total pressure is preferably made to be not more than4 Torr (532 Pa), more preferably not more than 2 Torr (266 Pa). From theviewpoint of preventing loss of a high-vapor-pressure metallic elementsuch as Pb, the waiting period between the second step and the thirdstep is preferably as short as possible, and hence the treatmentconditions (temperature and pressure) in the second step are preferablyclose to or the same as the treatment conditions in the third step, i.e.can be set as appropriate from within the ranges for the treatmentconditions in the third step.

Moreover, in the manufacturing method of the present invention, theconditions in the first step preferably satisfy at least one of thetemperature (the temperature of the conductive layer) being lower thanin the third step (hereinafter referred to as the ‘low temperaturenucleus formation condition’) and the raw material gas pressure beinghigher than in the third step (hereinafter referred to as the highpressure nucleus formation condition’). According to this method, thegrain size of the subsequently formed dielectric layer is reduced, andhence surface roughness is reduced. As a result, a dielectric film forwhich the leakage current is reduced, and the transparency is excellentand hence mask alignment can be carried out well is formed, and moreoverif such a dielectric film is applied to a capacitive element, then asemiconductor device having little variation in bit line voltagedifference can be manufactured. Here, from the viewpoint of reducingloss of a high-vapor-pressure metallic element such as Pb, the waitingperiod between the second step and the third step is preferably as shortas possible, and hence the conditions in the second step are preferablyclose to or the same as the conditions in the third step, i.e. can beset as appropriate from within the ranges for the treatment conditionsin the third step.

Regarding the low temperature nucleus formation condition, it ispreferable to set the temperature in the first step to be lower than thetemperature in the third step, with these temperatures being within thefollowing ranges. The pressures can be set within the previouslymentioned pressure ranges, or may be set in accordance with the highpressure nucleus formation condition described below.

Temperature in first step: Preferably at least 350° C., more preferablyat least 370° C.; on the other hand, preferably not more than 450° C.,more preferably not more than 400° C. Temperature in third step: Can beset as appropriate from within a range of 400 to 700° C.; preferably notmore than 470° C., more preferably not more than 450° C.

Regarding the high pressure nucleus formation condition, it ispreferable to set the pressure in the first step to be higher than thepressure in the third step, with these pressures being within thefollowing ranges. The temperatures can be set within the previouslymentioned temperature ranges, or can be set in accordance with the lowtemperature nucleus formation condition described above. Pressure infirst step: Preferably at least 0.1 Torr (13.3 Pa), more preferably atleast 1 Torr (133 Pa); on the other hand, preferably not more than 100Torr (13.3 kPa), more preferably not more than 20 Torr (2.66 kPa).Pressure in third step: Preferably at least 1×10⁻⁴ Torr (1.33×10⁻² Pa);on the other hand, preferably not more than 4 Torr (532 Pa), morepreferably not more than 2 Torr (266 Pa).

[Method of Manufacturing Semiconductor Storage Device]

Next, referring to FIG. 1, a description will be given of a method ofmanufacturing a semiconductor storage device having capacitive elements1 each comprising a dielectric layer, a lower electrode and an upperelectrode as described above.

First, a lower electrode 10 is formed on a first inter-layer insulatingfilm that has been provided on a semiconductor substrate on which activeelements such as transistors have been formed. Here, for example a TiNfilm or a multi-layered film of Ti and TiN (e.g. a Ti/TiN/Timulti-layered film) is formed as a barrier layer by sputtering, and thena conductive film of thickness approximately 100 nm comprising, forexample, Ru for lower electrode 10 formation is formed thereon bysputtering or CVD. Patterning for forming the lower electrodes may becarried out after the formation of this conductive film, or patterningof the lower electrodes may be carried out at a time after thisconductive film, a dielectric layer and a conductive film for upperelectrode formation have been formed. Moreover, the lower electrodes aredisposed so as to be electrically connected to plugs that are providedin the first inter-layer insulating film and are electrically connectedto the active elements.

Next, an initial nucleus 11, a buffer layer 12 and a dielectric layer 13are formed in this order using MOCVD in accordance with the methoddescribed earlier on the conductive film 10 for lower electrodeformation or the patterned lower electrodes.

Next, a conductive film 14 of thickness approximately 100 nm comprising,for example, Ru for upper electrode formation is formed on thedielectric layer 13 by sputtering or CVD.

After that, using dry etching, the barrier film, the lower electrodeconductive film 10, the dielectric layer 13 and the upper electrodeconductive film 14 are patterned, or in the case that the lowerelectrodes have already been formed, the dielectric film 13, the upperelectrode conductive film 14 and so on are patterned, whereby capacitiveelements each having an upper electrode, a lower electrode, and adielectric layer positioned between these electrodes are formed.

A second inter-layer insulating film is formed on the capacitiveelements formed as described above, plugs that are electricallyconnected to the upper electrodes are formed in the second inter-layerinsulating film, and then wiring that is electrically connected to theseplugs can be formed.

EXAMPLES

The present invention will be described in further detail throughexamples.

Example 1

On a 100 nm-thick Ru film as a lower electrode conductive film, usingraw materials as below with an MOCVD method as below, under conditionsof a substrate temperature of 430° C. and a film formation pressure of 1Torr (133 Pa), an initial nucleus (PTO, Ti/Pb atomic ratio=0.91), abuffer layer (PTO, Ti/Pb atomic ratio=0.63) and a ferroelectric film(PZT) were formed, and then a 100 nm-thick Au film was formed as anupper electrode using a vacuum deposition method.

-   -   Pb raw material: A solution of lead bis-dipivaloylmethanate        (Pb(DPM)₂) dissolved in an organic solvent (concentration 0.1        mol/L)    -   Ti raw material: A solution of titanium diisopropoxy        bis-dipivaloylmethanate (Ti(OiPr)₂(DPM)₂) dissolved in an        organic solvent (concentration 0.3 mol/L)    -   Zr raw material: A solution of zirconium isopropoxy        tris-dipivaloylmethanate (Zr(OiPr)(DPM)₃) dissolved in an        organic solvent (concentration 0.1 mol/L)    -   Oxidizing gas: Nitrogen dioxide (NO₂)        The Pb raw material, the Ti raw material and the Zr raw material        were transported as solutions, and then vaporized using a        vaporizer and supplied into the vacuum vessel (i.e. were        supplied using the so-called liquid transport method).

First, before forming the initial nucleus, together with 400 sccm ofnitrogen dioxide, the Pb raw material (0.3 ml/min) was supplied for 5seconds as a vaporized gas (pre-step), and then the Ti raw material(0.12 ml/min) was further supplied as a vaporized gas and this state washeld for 20 seconds, thus forming a 3 nm-thick initial nucleus(crystalline nucleus) (first step).

Next, the system was held for 20 seconds with the same temperature,pressure, Pb raw material supply rate and nitrogen dioxide supply rateas in the first step but with the Ti raw material supply rate reduced to0.06 ml/min, thus forming a 2 nm-thick buffer layer (second step).

Next, prescribed film formation conditions, namely a Pb raw materialsupply rate of 0.35 ml/min, a Ti raw material supply rate of 0.1 ml/min,and a Zr raw material supply rate of 0.21 ml/min, were changed to, andafter stabilization, the solutions of these raw materials were made intovaporized gases, and the vaporized gases were supplied for 900 secondstogether with 400 sccm of nitrogen dioxide, thus forming a 230 nm-thickferroelectric film (third step).

The hysteresis characteristics for the capacitive element formed asdescribed above are shown in FIG. 2A. It can be seen from this figurethat, compared with the comparative example described later, the lowvoltage characteristic, i.e. the remanent polarization at low voltage,is improved.

Note that FIG. 2A shows on top of one another the hysteresis curvesobtained for ±2 V, ±2.5 V, ±3 V, ±4 V and ±5 V bipolar single-shotvoltage sweeps (single-shot hysteresis curves).

Example 2

A capacitive element was formed as in Example 1, except that an oxide ofRu was formed as the upper electrode. The hysteresis characteristics forthe capacitive element formed are shown in FIG. 3A. It can be seen fromthis figure that, compared with the comparative example described later,the low voltage characteristic, i.e. the remanent polarization at lowvoltage, is improved.

Note that FIG. 3A shows on top of one another the hysteresis curvesobtained for ±2.5 V, ±3 V, ±4 V and ±5 V bipolar single-shot voltagesweeps (single-shot hysteresis curves).

Example 3

On a 100 nm-thick Ru film as a lower electrode conductive film, usingraw materials as below with an MOCVD method as below, an initial nucleus(PTO, Ti/Pb atomic ratio=1), a buffer layer (lead oxide, Ti/Pb atomicratio=0) and a ferroelectric film (PZT) were formed, and then a 100nm-thick Ru oxide film was formed as an upper electrode.

-   -   Pb raw material: Lead bis-dipivaloylmethanate (Pb(DPM)₂)    -   Ti raw material: Titanium isopropoxide (Ti(OiPr)₄)    -   Zr raw material: Zirconium butoxide (Zr(OtBu)₄)    -   Oxidizing gas: Nitrogen dioxide (NO₂)        The Pb raw material, the Ti raw material and the Zr raw material        were vaporized directly from a solid or liquid as is and        supplied into the vacuum vessel (i.e. were supplied using the        so-called solid sublimation method).

First, before forming the initial nucleus, at a substrate temperature of330° C. and a film formation pressure of 50 mTorr (6.65 Pa), togetherwith 20 sccm of nitrogen dioxide, 0.18 sccm of the Pb raw material wassupplied for 20 seconds (pre-step), and then at the same temperature andpressure the Ti raw material (0.24 sccm) was further supplied in andthis state was held for 10 seconds, thus forming a 2 nm-thick initialnucleus (crystalline nucleus) (first step).

Next, the substrate temperature was changed to 430° C., and this statewas held for 10 seconds with the same pressure, Pb raw material supplyrate and nitrogen dioxide supply rate as in the first step, thus forminga 0.4 nm-thick buffer layer (second step).

Next, prescribed film formation conditions, namely a Pb raw materialsupply rate of 0.18 sccm, a Ti raw material supply rate of 0.14 sccm, aZr raw material supply rate of 0.045 sccm, and a nitrogen dioxide supplyrate of 50 sccm were changed to, and after stabilization, these rawmaterials were supplied for 1250 seconds, thus forming a 230 nm-thickferroelectric layer (third step).

The hysteresis characteristics for the capacitive element formed asdescribed above are shown in FIG. 4A. It can be seen from this figurethat, compared with the comparative example described later, the lowvoltage characteristic, i.e. the remanent polarization at low voltage,is improved.

Note that FIG. 4A shows on top of one another the hysteresis curvesobtained for ±2.5 V, ±3 V, +4 V and ±5 V bipolar single-shot voltagesweeps (single-shot hysteresis curves).

Comparative Example 1

A capacitive element was produced as in Example 1, except that thesecond step was not carried out. The hysteresis characteristics for thecapacitive element formed are shown in FIG. 2B.

Comparative Example 2

A capacitive element was produced as in Example 2, except that thesecond step was not carried out. The hysteresis characteristics for thecapacitive element formed are shown in FIG. 3B.

Comparative Example 3

A capacitive element was produced as in Example 3, except that thesecond step was not carried out. The hysteresis characteristics for thecapacitive element formed are shown in FIG. 4B.

From the invention thus described, it will be obvious that theembodiments of the invention may be varied in many ways. Such variationsare not to be regarded as a departure from the spirit and scope of theinvention, and all such modifications as would be obvious to one skilledin the art are intended for inclusion within the scope of the followingclaims.

1. A method of manufacturing a semiconductor storage device having acapacitive element having a dielectric layer having a perovskite-typecrystal structure represented by general formula ABO₃ and a lowerelectrode and an upper electrode disposed so as to sandwich thedielectric layer therebetween, the method comprising: forming, on aconductive layer constituting the lower electrode, using a metal organicchemical vapor deposition method, an initial nucleus containing at leastone metallic element the same as a metallic element in the dielectriclayer; forming, on the initial nucleus, using a metal organic chemicalvapor deposition method, a buffer layer containing at least one metallicelement the same as the metallic element contained in both the initialnucleus and the dielectric layer, in a higher content than the contentof this metallic element contained in the initial nucleus; and forming,on the buffer layer, using a metal organic chemical vapor depositionmethod, the dielectric layer having a perovskite-type crystal structure.2. The method of manufacturing a semiconductor storage device accordingto claim 1, wherein the buffer layer contains lead as the metallicelement contained in a higher content than the content of the samecontained in the initial nucleus.
 3. The method of manufacturing asemiconductor storage device according to claim 1, wherein the bufferlayer is formed to a thickness such as not to exert an influence on thecrystal orientation of the dielectric layer.
 4. The method ofmanufacturing a semiconductor storage device according to claim 1,wherein the dielectric layer is a ferroelectric layer containing lead asan element A occupying the A lattice sites, and containing zirconium andtitanium as elements B occupying the B lattice sites.
 5. The method ofmanufacturing a semiconductor storage device according to claim 4,wherein the initial nucleus comprises lead titanate, and the bufferlayer comprises a metal oxide containing lead.
 6. The method ofmanufacturing a semiconductor storage device according to claim 5,wherein the buffer layer comprises lead titanate, and has a higher leadcontent than the lead content in the initial nucleus.
 7. The method ofmanufacturing a semiconductor storage device according to claim 5,wherein the buffer layer comprises lead oxide.
 8. The method ofmanufacturing a semiconductor storage device according to claim 1,wherein the conditions when forming the initial nucleus satisfy at leastone of a temperature being lower than when forming the dielectric layerand a raw material gas pressure being higher than when forming thedielectric layer.
 9. The method of manufacturing a semiconductor storagedevice according to claim 1, wherein the lower electrode has, at leaston a surface on a side of the dielectric layer, a film comprising atleast one material selected from platinum, iridium, ruthenium, andoxides thereof. 10-18. (canceled)